Strontium-modified zinc oxide voltage variable resistor

ABSTRACT

A VOLTAGE VARIABLE RESISTOR CERAMIC COMPOSITION CONSISTING ESSENTIALLY OF ZINC AND, AS AN ADDITIVE, STRONTIUM OXIDE, THE STRONTIUM-MODIFIED ZINC OXIDE VOLTAGE VARIABLE RESISTOR HAS IMPROVED VOLTAGE NONLINEAR PROPERTIES DUE TO THE FURTHER ADDITION OF BISMUTH OXIDE, LEAD OXIDE, CALCUIM OXIDE AND COBALT OXIDE.

Jan. 4, 1972 MICI-"O suo ETAL 3,632,529

STRONTIUM-MDIFIED ZINC OXIDE VOLTAGE VARABLE RESISTOR Filed Oct. 16,1969 INVENTORS MICHIO MATSUOKA TAKESH! MASUYAMA YOSHIO IJDA r ATTORNEYSUnted States Patent 3,632,529 STRONTIUM-MODIFIED ZINC OXIDE VOLTAGEVARIABLE RESISTOR Michio Matsuoka, Takeshi Masuyama, and Yoshio Iida,

Osaka-fu, Japan, assignors to Matsushita Electric Industrial Co., Ltd.,Kadoma, Osaka, Japan Filed Oct. 16, 1969, Ser. No. %6,819 Claimspriority, application Japan, Oct. 22, 1968, 43/77,735 Int. Cl. H01b 1/06U.S. Cl. 252-521 8 Claims ABSTRACT OF THE DISCLOSURE A voltage variableresistor ceramic composition consisting essentially of zinc oxide and,as an additive, strontium oxide. The strontium-modified zinc oxidevoltage variable resistor has improved voltage nonlinear properties dueto the further additon of bismuth oxide, lead oxide, calcium oxide andcobalt oxide.

This invention relates to compositions of voltage variable resistorceramics having non-ohmic resistance, and more particularly tocompositions of varistors comprising zinc oxide having non-ohmicresistance due to the bulk thereof.

Various voltage variable resistors such as silicon carbide varistors,selenium rectifiers and germanium or silicon p-n junction diodes havebeen widely used for stabilization of voltage or current of electricalcircuits. The elec- -trical characteristics of such a voltage variableresistor are expressed by the relation:

where V is the voltage 'across the resistor, I is the current flowingthrough the resistor, C is a constant corresponding to the voltage -at agiven current and exponent n is a numerical value greate; than 1. Thevalue of n is calculated by the following equation:

where V and V are the voltages at given currents I and Iz, respectively.The desired value of C depends upon the kind of application to which theresistor is to be put. It is ordinarily desirable that the Value of n beas large as possible since this exponent determines the extent to whichthe resistors depart from ohmic characteristics.

In conventional varistors comprising germanium or silicon p-n junctiondiodes, it is diflicult to control the C-value over a wide range becausethe voltage variable property of these Varistors is not attributed tothe bulk, but to the p-n junction. On the other hand, silicon ca'bidevaristors have voltage variable properties due to the contacts among theindividual grains of silicon carbide bonded together by a ceramicbinding material, and the C-value is controlled by changing a dimensionin a direction in which the current flows through the varistors. Siliconcarbide varistors, however, have a relatively low n-value and areprepared hy firing in non-oxidizing atmosphere, especially for thepurpose of obtaining a lower C-value.

An object of the present invention is to provide a composition of avoltage variable resistor having non-ohmic properties due to the bulkthereof and having a controllable C-value.

Another object of the present invention is to provide a composition of avoltage variable resistor characterized by a high n-value.

These and other objects of the invention will become apparent uponconsideration of the following description ice taken together with theaccompanying drawing in which the single figure is a partlycross-sectional View of a voltage variable resistor according to theinvention.

Before proceeding with a detailed description of the voltage variableresistors contemplated by the invention, their Construction will bedescribed with reference to the aforesaid figure of drawing whereinreference character 10 designates, as a whole, a voltage variableresistor compn'sing, as its active element, a sintered body having apair of electrodes 2 and 3 applied to opposite surfaces thereof. Saidsintered body 1 is prepared in a manner hereinafter set forth and is inany form such as circular, square or rectangular plate form. Wire leads5 and `6 are attached conductively to the electrodes 2 and 3,respectively, by a connection means 4 such as solder or the like.

A voltage variable resistor according to the invention comprises asintered body of a composition consisting essentially of 90.0 to 99.95mole percent of zinc oxide and 0.05 to 10.0 mole percent of strontiumoxide. Such a voltage variable resistor has non-ohmic resistance due tothe bulk itself. Therefore, its C-value can be changed without impairingthe n-value by changing the distance between said opposite surfaces. Theshorter distance results in the lower C-value.

The higher n-value can be obtained when said sintered body consistingessentially of 97.0 -to 99.9 mole percent of zinc oxide and 0.1 to 3.0mole percent of strontium oxide in accordance with the invention.

According to the present invention, the C-value can be lowered withoutchanging the dimension or lowering n-value when said sintered body is ofa composition consisting essentially of 82.0 to 99.9 mole percent ofzinc oxide, 0.05 to 10.0 mole percent of strontium oxide and 0.05 to 8.0mole percent of bismuth oxide.

A combination of a low C-value and a high n-value can be obtained whensaid sintered body consists essentially of 94.0 to 99.8 mole percent ofzinc oxide, 0.1 to 3.0 mole percent of strontium oxide and 0.1 to 3.0mole percent of bismuth oxide.

According to the present invention, the stability for ambienttemperature and the electric load life test can be improved when saidsintered body consists essentially of 82.0 to 99.9 mole percent of zincoxide, 0.05 to 10.0 mole percent of strontium oxide and 0.05 to 8.0 molepercent of calcium oxide.

Further, the stability for ambient temperature and the electric loadlife test is extremely improved when said sintered body consistsessentially of 94.0 to 99.8 mole percent of zinc oxide, 0.1 to 3.0 molepercent of strontium oxide and 0.1 to 3.0 mole percent of calcium oxide.

According to the present invention, the n-value is elevated when saidsintered body consists essentially of 82.0 to 99.9 mole percent of zincoxide, 0.05 to 10.0 mole percent of strontium oxide and 0.05 to 8.0 molepercent of one oxide selected from the group consisting of lead oxideand cobalt oxide.

The n-value is further elevated when said sintered body is of acomposition consisting essentially of 94.0 to 99.8 mole percent of zincoxide, 0.1 to 3.0 mole percent of strontium oxide and 0.1 to 3.0 molepercent of one oxide selected from the group consisting of lead oxideand cobalt oxide.

According to the present invention, the resistor has the high n-valuewhen said sintered body is of a composition consisting essentially of74.0 to 99.85 mole percent of zinc oxide, 0.05 to 10.0 mole percent ofstrontium oxide, 0.05 to 8.0 mole percent of lead oxide and 0.05 to 8.0mole percent of cobalt oxide.

The n-value is extremely elevated when said sintered body is of acomposition consisting essentially of 94.0 to 99.8 mole percent of zincoxide, 0.1 to 3.0 mole percent of strontium oxide, 0.1 to 3.0 molepercent of lead oxide and 0.1 to 3.0 mole percent of cobalt oxide.

According to the present invention, both the high nvalue and the lowC-value can be obtained when said sintered body is of a compositionconsisting essentially of 74.0 to 99.85 mole percent of zinc oxide, 0.05to 10.0 mole percent of strontium oxide, 0.05 to 8.0 mole percent ofbsmuth oxide and 0.05 to 8.0 mole percent of cobalt oxide.

Further, the C-value is lowered and the n-Value is extremely elevatedwhen said sintered body is of a composition consisting essentially of91.0 to 99.7 mole percent of zinc oxide, 0.1 to 3.0 mole percent ofstrontium oxide, and 0.1 to 3.0 mole percent of bsmuth oxide and 0.1 to3.0 mole percent of cobalt oxide.

According to the present invention, both the high nvalue and the lowC-value also can be obtained when said sintered body is of a compositionconsisting essentially of 74.0 to 99.7 mole percent of zinc oxide, 0.05to 10.0 mole percent of strontium oxide, 0.05 to 8.0 mole percent ofbsmuth oxide and 0.05 to 8.0 mole percent of lead oxide.

Further, a combination of extremely high n-value and the low C-valuealso can be obtained when said sintered body is of a compositionconsisting essentially of 91.0 to 99.7 mole percent of zinc oxide, `0.1to 3.0 mole percent of strontium oxide, 0.1 to 3.0 mole percent ofbsmuth oxide and 0.1 to 3.0 mole percent of lead oxide.

The sintered body 1 can be prepared by a per se well known ceramictechnique. The starting materials of the compositions described in theforegoing description are mixed in a wet mill so as to producehomogeneous mixtures. The mixtures are dried and pressed in a mold intodesired shapes at a pressure from 100 kg./cm. to 1000 kg./cm. Thepressed bodies are sintered in air at a given temperature for 1 to 3hours, and then furnace-cooled to room temperature (about 15 to about 30C.).

The available sintering temperature is determined in view of electricalresistvity, non-linearity and stability and ranges from 1000 to 1450 C.

The pressed bodies are preferably sintered in non-oxidizing atmospheresuch as nitrogen and argon when it is desired to reduce the electricalresistivity.

The mixtures can be preliminarily calcined at 700 to 1000 C. andpulverized for easy fabrication in the subsequent pressing step. Themixture to be pressed can be admixed with a suitable binder such aswater, polyvinyl alcohol, etc.

It is advantageous that the sintered body be lapped at the oppositesurfaces by abrasive powder such as silicon carbide in a particle sizeof 300 meshes to 1500 meshes.

The sintered bodies are provided, at the opposite surfaces thereof, withelectrodes in any available and suitable method such as electroplatngmethod, vacuum evaporatior method, metallizing method by spraying orsilver painting method.

The voltage variable properties are not practically affected by thekinds of electrodes used, but are affected by the thickness of thesintered bodies. Particularly, the C- value varies in proportion to thethickness of the sintered bodies, while the n-value is almostindependent of the thickness. This surely means that the voltagevariable property is due to the bulk of the body, but not to theelectrode.

Lead wires can be attached to the electrodes in a per se conventionalmanner by using conventional solder having a low melting point. It isconvenient to employ a conductive adhesive comprising silver powder andresin in an Organic solvent in order to connect the lead wires to theelectrodes.

Voltage Variable resistors according to this invention have a highstability to temperature and to the load life test, which is carried outat 70 C. at a rating power for 500 hours. The -value and C-value do notchange remarkably after heating cycles and load life test. It is ad- 4vantageous for achievement of a high stability to humidity that theresultant voltage variable resistors are embedded in a humidity proofresin such as epoxy resin and phenol resin in a per se well knownmanner.

Presently preferred illustrative embodments of the invention are asfollows.

EXAMPLE 1 A mixture of zinc oxide and strontium oxide in a compositionof Table 1 is mixed in a wet mill for 3 hours. The mixture is dried andthen calcined at 700 C. for 1 hour. The calcined mxture is pulverized bythe motor-driven ceramic mortar for 30 minutes and then pressed in amold into a shape of 17.5 mm. in diameter and 2.5 mm. in thickness at apressure of 500 kg./cm.

The pressed body is sintered in air at 1350 C. for 1 hour, and thenfurnace-cooled to room temperature (about 15 to about 30 C.). Thesintered disc is lapped at the opposite surfaces thereof by siliconcarbide in a particle size of 600 meshes. The resulting sintered dischas a size of 14 mm. in diameter and 1.5 mm. in thickness. The silverpaint electrodes commercially available are attached to the oppositesurfaces of sintered disc by painting. Then lead wires are attached tothe silver electrodes by soldering. The electric characteristics of theresultant resistors are shown in Table 1. It will be readily understoodthat the zinc oxide sintered body incorporated with strontium oxide inan amount of 0.05 to 10.0 mole percent is available for the voltagevariable resistor. Particularly, the addition of strontium oxide in anamount of 0.1 to 3.0 mole percent makes the voltage nonlinear propertymore excellent.

TABLE 1 SrO C SrO C (mole percent) (at 1 ma.) (mole percent) (at 1 Ina.)n

EXAMPLE 2 Starting materials composed of 99.5 mole percent of zinc oxideand 0.5 mole percent of strontium oxide are mixed, dried, calcined andpulverized in the same manner as those of Example 1. The pulverizedmixture is pressed in a mold into a disc of 17.5 mm. in diameter and 5mm. in thickness at a pressure of 500 kg./cm.

The pressed body is sintered in air at 1350 C. for 1 hour, and thenfurnace-cooled to room temperature. The sintered disc is ground at theopposite surfaces thereof into the thickness shown in Table 2 by siliconcarbide in a particle size of 600 meshes. The ground disc is pro videdwith the electrodes and lead wires at the opposite surfaces in a mannersimilar to that of Example 1. The electric characteristics of theresultant resistors are shown in Table 2; the C-value variesapproximately in proportion to the thickness of the sintered disc whilethe n-value is essentially independent of the thickness. It will bereadily realized that the voltage nonlinear property of the resistorsare attributed to the sintered body itself.

TABLE 2 C (at 1 ma.)

'l`lickncss, mm.

Initial (4.1) 3.5

EXAMPLE 3 Zinc oxide incorporated with strontium oxide and bismuth oxidein the composition of Table 3 is fabricated into the voltage varableresistors by the same process as that of Example 1. The resultingproperties are shown in Table 3. It can be easily understood that thecombination of strontium oxide and bismuth oxide as additive effectivelylowers the C-value without appreciably changing the n-value.

TABLE 3 Mole percent C SrO 131 (at l ma.) 'n

O. 0.05 200 3. 2 0. 05 0.5 163 3.3 0. 05 8 195 3. 4 0. 5 0. 05 18 5. 00. 5 8 18 5. 0 0. 05 170 3. 1 10 0. 5 50 3. 3 10 8 168 3. 3 0. 1 0. 1 814. 1 0. 1 0. 5 32 4. 2 0. 1 3 81 4. 0 0. 5 0. 1 14 4. 9 0, 5 3 13 5. 1 30. 1 35 4. O 3 0.5 4. 2 3 3 34 4. 1 0. 5 O. 5 6 5. 2

EXAMPLE 4 Zinc oxide incorporated with strontium oxide and calcium oxidein the composition of Table 4 is fabricated into voltage varableresistors by the same process as that of Example 1. The resultingresistors are tested according to the methods used in the electronicComponents parts. The load life test is carried out at 70 C. ambienttemperature at 0.5 watt rating power for 500 hours. The heating cycletest is carried out by repeating 5 times the cycle in which saidresistors are kept at 85 C. ambient' temperature for 30 minutes, cooledrapidly to C. and then kept at such temperature for 30 minutes. Table 4shows a difference in the C-value and n-value between resistors beforeand after the load life test. It can be readily realzed that thecombination of strontium oxide and calcum oxide as additive is effectivefor electrical and environmental stability.

Heating circle Test Mole percent AC An AC An SrO Cao (percent) (percent)(percent) (percent) 0. 05 0. 05 -8. 2 -7. 2 -7. 9 -8. 0 0. 05 0. 5 4. 74. 2 -5. 8 -5. 2 0. 05 8 -6. 0 -5. 7 -7. 6 -6. 4 0.5 0.05 -5.4 -5.0 -4.2-5.3 0.5 8 -5. 0 -:3.4 -4.3 -5.5 10 O. 05 -7. 1 -6. 3 7. 4 -7. 4 10 0.55. 3 5. 0 6. 0 -6. 1 10 8 -8. 0 -6. 9 -7. 7 -8. 4 0. 1 0.1 -5. 0 3. 6-3. 8 5. 0 0. 1 0. 5 3. 9 --2. 2 -3. 7 -3. 9 0. 1 3 -4. 3 -2. 5 -3. 0-4. 7 0.5 0.1 -2.8 1.8 -2.8 3.7 0. 5 3 -2.9 -1.7 2.4 5. 2 3 O. l 4. 0-2, 4 -4. 7 5. 0 3 0.5 -3. 2 -1.8 -3. 0 -4. 1 3 3 -4.9 -3. 4 -5. 1 -4, 90. 5 0.5 -0. 1 -0, 2 -0. 2 --0.1

EXAMPLE 5 Zinc oxide containing the additions of Table 5 is fabricatedinto 'voltage varable resistors by the same process as that ofExample 1. The n-values of the resulting resistors are shown in Table 5.It will be readily seen that the combnation of strontium oxide with leadoxide and/ or cobalt oxide as additives results in a remarkablyexcellent voltage-nonlinear property.

TABLE 5 Mola percent PbO C CoO (at 1 ma.)

EXAMPLE 6 TABLE 6 Mole percent PbO What is claimed is:

1. A voltage varable resistor cera mic composition con sistingessentially of zinc oxide and 0.05 to 10.0 mole percent of strontiumoxide.

2. A voltage varable resistor ceramic composition as claimed in claim 1,wherein said composition consists essentially of zinc oxide and 0.1 to3.0 mole percent of strontium oxide.

3. A voltage variable resistor ceramic composition as claimed in claim 1wherein said composition further includes 0.05 to 8.0 mole percent ofone oxide selected from the group consisting of bismuth oxide, leadoxide, calcium oxide and cobalt oxide.

4. A voltage variable resistor ceramic composition as claimed in claim 2wherein said composition further includes 0.1 to 3.0 mole percent of oneoxide selected from the group consisting of .bismuth oxide, lead oxide,calcium oxide and cobalt oxide.

5. A voltage variable resistor ceramic composition as claimed in claim 1wherein said composition further includes 0.05 to 8.0 mole percent ofcobalt oxide and 0.05 to 8.0 mole percent of one oxide selected from thegroup consisting of bismuth oxide and lead oxide.

6. A voltage variable resistor ceramic composition as claimed in claim 2wherein said composition further ircludes 0.1 to 3.0 mole percent ofcobalt oxide and 0.1

8 to 3.0 mole percent of one oxide selected from the group consisting ofbismuth oxide and lead oxide.

7. A voltage varable resistor ceramic composition as claimed in claim 1wheren said composition further ncludes 0.05 to 8.0 mole percent of leadoxide and 0.05 to 8.0 mole percent of bismuth oxide.

8. A voltage varable resistor ceramic composition as claimed in claim 2wherein said composition further includes 0.1 to 3.0 mole percent oflead oxide and 0.1 to 3.0 mole percent of bismuth oxide.

References Cited UNITED STATES PATENTS 2,857,294 10/1958 Davis 252--5182,892,988 6/1959 Schusterius 252-520 DOUGLAS J. DRUMMOND, PrimaryExaminer

